This is your badge of honor. The dress jacket is the official uniform of the working woman, and you'll want to keep one very close during your first days in the office. Until you've established a rapport with your coworkers, blending in is your best office fashion strategy -- and there's no better camouflage than a dress jacket.
Brown works, and beige is possible, but a black jacket is perfect for something simple and elegant. In case of emergencies, keeps a spare black jacket hanging from the back of your chair. If an impromptu meeting with the boss comes up, you'll be able to stride in confidently under a good pair of shoulder pads.